U.S. Geological Survey Groundwater Toolbox, A Graphical 

and Mapping Interface for Analysis of Hydrologic Data 

(Version 1.0)—User Guide for Estimation of Base Flow, 

Runoff, and Groundwater Recharge From Streamflow Data

Techniques and Methods

Book 3

Chapter B10

U.S. Department of the Interior

U.S. Geological Survey
Cover. Screen captures of the U.S. Geological Survey Groundwater Toolbox showing a map of streamflow and groundwater-level datacollection locations (top), estimated monthly recharge (middle), and flow-duration curves for measured streamflow and estimated base flow 

and runoff (bottom) for the Pawcatuck River Basin at Wood River Junction, Rhode Island, streamgage.
U.S. Geological Survey Groundwater 

Toolbox, A Graphical and Mapping 

Interface for Analysis of Hydrologic Data 

(Version 1.0)—User Guide for Estimation 

of Base Flow, Runoff, and Groundwater 

Recharge From Streamflow Data

By Paul M. Barlow, William L. Cunningham, Tong Zhai, and Mark Gray

Techniques and Methods

Book 3

Chapter B10

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov/ or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod/

To order this and other USGS information products, visit http://store.usgs.gov/

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Barlow, P.M., Cunningham, W.L., Zhai, Tong, and Gray, Mark, 2015, U.S. Geological Survey Groundwater Toolbox, a 

graphical and mapping interface for analysis of hydrologic data (version 1.0)—User guide for estimation of base flow, 

runoff, and groundwater recharge from streamflow data: U.S. Geological Survey Techniques and Methods, book 3, 

chap. B10, 27 p., http://dx.doi.org/10.3133/tm3B10.

ISSN 2328-7055 (online)
iii

Preface

This report is a user guide for the streamflow-hydrograph analysis methods provided with version 1.0 of the U.S. Geological Survey Groundwater Toolbox. These include six hydrograph-separation methods to determine the groundwater-discharge (base-flow) and surface-runoff components of streamflow—the Base-Flow Index (BFI; Standard and Modified), HYSEP (Fixed Interval, 

Sliding Interval, and Local Minimum), and PART hydrograph-separation methods—and the RORA 

recession-curve displacement method and supporting RECESS program to estimate groundwater 

recharge from streamflow data. The performance of the program has been tested in a variety of 

applications, some of which are documented in this report. Future applications, however, might 

reveal errors that were not detected in the test simulations. Users are requested to notify the 

U.S. Geological Survey (USGS) of any errors found in this report or the computer program.

Although this computer program has been written and used by the USGS, no warranty, 

expressed or implied, is made by the USGS or the U.S. Government as to the accuracy and functionality of the program and related program material, nor shall the fact of distribution constitute 

any such warranty, and no responsibility is assumed by the USGS in connection therewith. The 

Groundwater Toolbox and other groundwater programs are available online from the USGS at 

http://water.usgs.gov/software/lists/groundwater/. Information on how to download and install 

the program can be found at that Web site.
iv

Acknowledgments

We thank our U.S. Geological Survey (USGS) colleagues David Nelms and Richard Healy for 

their review of the Groundwater (GW) Toolbox and an earlier draft of this report. We also thank 

Leslie DeSimone and Anna Glover, USGS, for their editorial comments on the draft report. David 

Nelms and Ronald Sloto of the USGS and Tony Wahl of the Bureau of Reclamation did extensive 

testing of successive draft versions of the GW Toolbox; their reviews identified errors in the 

implementation of the hydrograph-analysis methods in the GW Toolbox that could be corrected 

before release of the code. Albert Rutledge, USGS (retired), also provided several helpful review 

comments on the PART, RORA, and RECESS programs described in this report. We also thank 

several other of our USGS colleagues who tested the GW Toolbox and provided feedback on the 

software.
v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Base-Flow, Runoff, and Recharge Estimation Methods and Applicability..................................2

Example Basin.......................................................................................................................................5

Getting Started ...............................................................................................................................................5

Building, Saving, and Reopening a Project ......................................................................................5

Time-Series Data Discovery and Retrieval.......................................................................................7

Summarizing, Listing, and Graphing Time-Series Data ................................................................11

Meteorological Data...........................................................................................................................13

Base-Flow and Runoff Components of Streamflow: Hydrograph Separation...................................16

Groundwater Recharge ..............................................................................................................................17

RECESS Program To Estimate Basin Recession Index.................................................................17

RORA Program To Estimate Groundwater Recharge....................................................................21

Water Budgets for the Example Basin .....................................................................................................21

Summary........................................................................................................................................................25

References Cited..........................................................................................................................................25

Figures

1. Guidance on appropriateness of hydrograph-analysis methods provided with the

Groundwater Toolbox...................................................................................................................4

2. Screen captures showing the “Welcome to the USGS GW Toolbox 1.0” and “USGS

GW Toolbox 1.0” dialog boxes that appear on first use of the Groundwater Toolbox ......6

3. Screen captures showing the initial map with data layers active that show states

and hydrologic cataloging units. ...............................................................................................7

4. Screen captures showing the method for adding labels to a map layer. In this

example, cataloging-unit numbers are added to the Cataloging Units layer. A,

Options available by right-clicking on the selected layer, “Cataloging Units.” B,

“Label-style” dialog box that appears after selecting the “Label Setup” option, and,

C, The resulting map, with cataloging-unit labels added.......................................................8

5. Screen captures showing selected steps to build a Groundwater Toolbox project.

A, The “Select” tool is used to select the cataloging unit (basin) of interest, which

is then shaded yellow. B, The resulting map, which includes daily streamflow

(discharge) and groundwater-level data-collection locations for the basin of

interest and surrounding area..................................................................................................10

6. Screen capture showing steps used to select one or more data sites for which

data can be retrieved from the U.S. Geological Survey National Water Information

System for use in the Groundwater Toolbox. ........................................................................11

7. Screen captures showing dialog boxes used to retrieve data from the U.S.

Geological Survey National Water Information System and other sources for use in

the Groundwater Toolbox. A, The “Download Data” dialog box used to retrieve

streamflow (discharge) data. B, The dialog box indicating that the data for the site

of interest (01117500) have been retrieved and are available for use...............................12
vi

8. Screen captures showing dialog boxes used to select and graph data for use in the

Groundwater Toolbox. A, The “Select Data to Graph” dialog box for selecting

time-series data to graph. B, The “Choose Graphs to Create” dialog box to select

graph types. ................................................................................................................................14

9. Screen captures showing graphs of streamflow data at the example streamgage

on the Pawcatuck River at Wood River Junction, Rhode Island, for use in the

Groundwater Toolbox. A, Streamflow hydrograph. B, Flow-duration curve ....................15

10. Screen captures showing dialog boxes used in the base-flow separation analysis

component of the Groundwater Toolbox. A, The “Base-Flow Separation” dialog

box. B, Part of the “Data Summary” dialog box, which shows months having

complete (dots) or incomplete (Xs) records...........................................................................17

11. Screen capture showing the flow-duration curves for measured streamflow and

estimated base flow and runoff for the Pawcatuck River at Wood River Junction,

Rhode Island, from 1971 through 2000 using the six hydrograph-separation methods

available in the Groundwater Toolbox.....................................................................................18

12. Screen captures showing dialog boxes used for the RECESS component of the

Groundwater Toolbox. A, Initial dialog box to specify the dates, months, and

seasons for analysis, the minimum number of days required for detection of a

recession period, and the output directory. B, Dialog box showing recession

periods that meet the minimum-day requirement (left side of dialog box), the first

recession period beginning after a peak streamflow on January 7, 1971 (middle

part of dialog box), and a semilogarithmic graph of the first recession period (right

side of dialog box). C, Dialog box showing the estimated recession index of

38.432305 days per log cycle for full period of recession from January 8 through

January 22, 1971, D, The “Choose Recession Limb Duration” dialog box allows the

user to refine the days that will be used to calculate the recession index. E, Dialog

box showing a summary, the median value, and a scatter plot of the recession

indices calculated by RECESS for the example basin. ........................................................19

13. Screen capture showing the “USGS RORA” dialog box to estimate groundwater

recharge by use of the RORA program in the Groundwater Toolbox ................................22

14. Screen capture showing graph of monthly recharge, in inches over the drainage

basin, calculated by the RORA program for the Pawcatuck River Basin at Wood

River Junction, Rhode Island, from January 1971 through December 2000 .....................23

Table

1. Water-budget components estimated for the Pawcatuck River Basin at Wood River

Junction, Rhode Island from 1971 through 2000....................................................................23
vii

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

inch per year (in/yr) 2.54 centimeter per year (cm/yr)

square mile (mi2

) 2.590 square kilometer (km2) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × 

°C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as °C = (°F – 

32) / 1.8.

Abbreviations

BASINS Better Assessment Science Integrating Point and Nonpoint Sources 

environmental analysis system

BFI Base-Flow Index method

EPA U.S. Environmental Protection Agency

GIS geographic information system

GW groundwater

NOAA National Oceanic and Atmospheric Administration

NCDC National Climatic Data Center

NWIS National Water Information System

USGS U.S. Geological Survey

U.S. Geological Survey Groundwater Toolbox, A Graphical 

and Mapping Interface for Analysis of Hydrologic Data 

(Version 1.0)—User Guide for Estimation of Base Flow, 

Runoff, and Groundwater Recharge From Streamflow Data

By Paul M. Barlow,1 William L. Cunningham,1 Tong Zhai,2 and Mark Gray2

1

 U.S. Geological Survey

2

 AQUA TERRA Consultants

Abstract

This report is a user guide for the streamflow-hydrograph 

analysis methods provided with version 1.0 of the U.S. 

Geological Survey (USGS) Groundwater Toolbox computer 

program. These include six hydrograph-separation methods to 

determine the groundwater-discharge (base-flow) and surfacerunoff components of streamflow—the Base-Flow Index (BFI; 

Standard and Modified), HYSEP (Fixed Interval, Sliding 

Interval, and Local Minimum), and PART methods—and the 

RORA recession-curve displacement method and associated 

RECESS program to estimate groundwater recharge from 

streamflow data. The Groundwater Toolbox is a customized 

interface built on the nonproprietary, open source MapWindow geographic information system software. The program 

provides graphing, mapping, and analysis capabilities in a 

Microsoft Windows computing environment. In addition 

to the four hydrograph-analysis methods, the Groundwater 

Toolbox allows for the retrieval of hydrologic time-series 

data (streamflow, groundwater levels, and precipitation) from 

the USGS National Water Information System, downloading of a suite of preprocessed geographic information system 

coverages and meteorological data from the National Oceanic 

and Atmospheric Administration National Climatic Data 

Center, and analysis of data with several preprocessing and 

postprocessing utilities. With its data retrieval and analysis 

tools, the Groundwater Toolbox provides methods to estimate 

many of the components of the water budget for a hydrologic basin, including precipitation; streamflow; base flow; 

runoff; groundwater recharge; and total, groundwater, and 

near-surface evapotranspiration.

Introduction

Analysis of streamflow hydrographs has long been used 

to identify the groundwater-discharge (or base-flow) and 

surface-runoff components of streamflow, as well as to estimate basinwide rates of groundwater recharge. These analyses not only provide insight to the relative contributions of 

groundwater discharge and surface runoff to streams, but also 

help to quantify the groundwater components of basin-scale 

hydrologic budgets. Many hydrograph-separation methods 

have been developed to estimate the base-flow and runoff 

components of streamflow and, in recent years, these methods 

have been implemented in a number of computer programs 

that facilitate the estimation process (Nathan and McMahon, 

1990; Wahl and Wahl, 1995; Sloto and Crouse, 1996; Rutledge, 1998; Arnold and Allen, 1999; Eckhardt, 2005; Lim 

and others, 2005; Piggott and others, 2005). Although each of 

the methods is based on formalized algorithms for identifying 

the base-flow component of total streamflow, the methods are 

subjective and not based on mathematical solutions to groundwater- or overland-flow equations. As a result, it is advantageous to use more than one hydrograph-separation method to 

analyze a streamflow record and then to compare the results of 

the multiple methods.

A method also has been developed to estimate groundwater recharge from streamflow hydrographs. Unlike the empirical hydrograph-separation methods, the recharge-estimation 

method, which is referred to as the recession-curve displacement method (Rorabaugh, 1964), is based on a mathematical 

solution to a one-dimensional form of the groundwater-flow 

equation for conditions of an instantaneous rise in the height 

of the water table over the basin.

Although several computer programs have been written for hydrograph separation and the estimation of groundwater recharge, the programs must be run individually and 

their results, which often are presented in different formats, 

compared manually. Moreover, each of the existing programs 

must be maintained and supported, and in some cases have 

become outdated and difficult to use. As a result, the U.S. 
2 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Geological Survey (USGS) Office of Groundwater determined 

that it would be useful to develop a single program in which a 

suite of application tools, including data retrieval and multiple 

hydrograph-separation and recharge-estimation methods, 

would be available for use in a modern computing environment. The USGS collaborated under contract with AQUA 

TERRA Consultants to develop the Groundwater (GW) Toolbox, which is a customized interface built on the MapWindow 

geographic information system (GIS) software (MapWindow, 

2013). MapWindow is a nonproprietary, open-source program 

that provides GIS capabilities in a graphical user interface and 

runs in a Microsoft Windows computing environment. The 

GW Toolbox has a similar interface and many of the same 

utilities as the Better Assessment Science Integrating Point and 

Non-point Sources (BASINS) environmental-analysis system 

developed by the U.S. Environment Protection Agency and 

AQUA TERRA Consultants (U.S. Environmental Protection 

Agency, 2013).

In addition to the several hydrograph-analysis methods, 

the GW Toolbox includes functions that retrieve hydrologic 

time-series data (streamflow, groundwater levels, and precipitation) from the USGS National Water Information System 

(NWIS) Web interface (U.S. Geological Survey, 2013), that 

download preprocessed GIS data layers and meteorological 

data from the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC), 

and that analyze data with several preprocessing and postprocessing utilities. The data retrieval and analysis tools 

provided by the GW Toolbox also can be used to estimate 

several of the components of the water budget for a hydrologic basin, including precipitation; streamflow; base flow; 

runoff; groundwater recharge; and total, groundwater, and 

near-surface evapotranspiration.

The primary purpose of this report is to provide a set 

of instructions that will allow users of the GW Toolbox to 

quickly develop skills to use its mapping, data-retrieval, datainspection, and hydrograph-analysis tools to estimate base 

flow, surface runoff, and groundwater recharge for a hydrologic basin. A secondary objective is to illustrate how the GW 

Toolbox can be used to develop a water budget for basins that 

have little to no groundwater withdrawals or streamflow regulation. The report supplements information that is available in 

the ‘GW Toolbox Documentation’ manual provided with the 

GW Toolbox. The online documentation is an integral part of 

the GW Toolbox software. In several cases, there are alternative approaches for completing work tasks; these alternatives 

can be found in the documentation.

Base-Flow, Runoff, and Recharge Estimation 

Methods and Applicability

Version 1.0 of the GW Toolbox includes six hydrograph-separation methods to calculate base flow—the BaseFlow Index (BFI; Standard and Modified), HYSEP (Fixed 

Interval, Sliding Interval, and Local Minimum), and PART 

methods—and a single recession-curve displacement method 

to calculate groundwater recharge—the RORA method. Each 

of the four computer programs in which the methods were 

originally implemented were reprogrammed from their original Fortran language into the Visual Basic.Net language within 

Microsoft Visual Studio .NET.

Each method uses a time series of daily mean streamflow (discharge) measured at a streamgage as the basis for 

estimating either base flow or recharge. Surface-water runoff 

is calculated as the difference between total streamflow and 

the calculated base flow. Each method requires specification 

of the drainage area of the basin (A, in square miles), which 

is used to convert volumetric flow rates of streamflow, base 

flow, runoff, and recharge (in cubic feet per second) to flow 

rates per unit area over the contributing drainage area (in 

inches). Drainage area also is needed for the HYSEP, PART, 

and RORA methods to calculate the duration of surface 

runoff for the basin (Nsr, in days; Sloto and Crouse, 1996; 

Rutledge, 1998):

 Nsr = A0.2. (1)

For convenience, the GW Toolbox will automatically retrieve 

the drainage area of the streamgage if it is available as an 

attribute of the streamgage in NWIS.

The seven hydrograph-analysis methods, as well as any 

changes that were made to the programs for implementation 

in the GW Toolbox, are described briefly here; each method is 

described in detail in the references cited. A full understanding of these references is integral to the proper application of 

the methods. Those readers unfamiliar with the methods in 

the GW Toolbox should review the relevant references before 

applying the methods.

BFI.—The BFI program (Wahl and Wahl, 1995) is based 

on a set of procedures developed by the Institute of Hydrology 

(1980a, b) in which the streamflow record is partitioned into 

intervals of length N-days. The minimum streamflow during 

each N-day interval then is identified and compared to adjacent minimums to determine “turning points.” If 90 percent 

of a given minimum (the “turning point test factor”) is less 

than both adjacent minimums, then that minimum is a turning 

point. The base-flow hydrograph is completed by connecting 

the turning points. The original BFI program set N to 5 days; 

the current version allows the user to vary the values of N

and the turning point test factor (f) to “...permit tuning the 

algorithm for different watersheds or to match other base-flow 

separation methods” (Wahl and Wahl, 1995). In the BFI-modified approach, parameter f is replaced by a daily recession 

index K′, and the turning-point test considers the exact number 

of days between turning-point candidates. Results obtained 

by the modified approach will usually be very similar to those 

obtained by the standard approach if

  1

K' f  =   N

. (2)
Introduction 3

For values of N = 5 days and f = 0.9, the equivalent value of K′ 

is 0.979 (Tony L. Wahl, Bureau of Reclamation, written commun., 2012). One modification was made to the BFI program 

for implementation in the GW Toolbox. In the original program, turning points were identified for each individual year, 

with partitioning restarted on day 1 of each year, and artificial 

(or virtual) turning points were created at the yearend boundaries. In the GW Toolbox implementation, turning points are 

identified continuously throughout the entire period of record, 

which avoids the creation of artificial turning points at the end 

of each year. This modification also changes how the daily 

values are partitioned after a year is completed in which the 

number of days in the year is not an even multiple of N. In 

test problems, the modification generally produced very small 

differences in calculations between the original program and 

the implementation of the program in the GW Toolbox when 

short time periods are considered; these differences increased, 

however, but were still generally small, as the number of years 

analyzed increased because of increasingly divergent values of 

the turning points selected by the two approaches.

HYSEP.—The HYSEP software (Sloto and Crouse, 1996) 

uses three methods originally developed by Pettyjohn and 

Henning (1979) to separate base-flow and runoff components 

of a streamflow hydrograph: the fixed interval, sliding interval, 

and local minimum methods. The three methods use different 

algorithms to draw connecting lines (the base-flow hydrograph) between low points of the streamflow hydrograph. 

No changes were made to the HYSEP algorithms for the 

GW Toolbox.

PART.—The PART program (Rutledge, 1998) equates 

base flow to streamflow on days that are designated as being 

unaffected by surface runoff or interflow (stormflow), and linearly interpolates between these days to determine base flow 

for the remainder of the hydrograph. Days that are unaffected 

by surface runoff are identified by the program as those preceded by Nsr days of continuous recession (see Rutledge, 1998, 

p. 34–36; Rutledge, 2007). One small change was made to 

PART for implementation in the GW Toolbox. In the original 

program, each daily value of base flow was calculated by running the program three times for three different values of Nsr; 

a curvilinear interpolation of the three calculated values was 

then done to determine the final value of base flow for each 

day (see Rutledge, 1998, p. 34–35). Although the GW Toolbox 

still runs PART three times for each streamflow record (and 

reports the values for each run in output file partday.txt), only 

the results of the first two values of Nsr are used to calculate 

the final, reported value of base flow on each day (in output 

file daily.csv). This was done for consistency with the method 

used by the program to calculate monthly and annual values of 

base flow.

RORA and RECESS.—The RORA program uses the 

recession-curve displacement method developed by Rorabaugh (1964) to estimate groundwater recharge for each peak 

in a streamflow hydrograph (Rutledge and Daniel, 1994; 

Rutledge, 1998). The method is based on a mathematical 

expression for the total volume of water that will drain from 

an aquifer following each recharge event. RORA requires that 

the user specify a recession index, K, for the basin. K can be 

determined in practice as the time required for groundwater 

discharge to decline through one log cycle on a semilogarithmic plot of the logarithm of streamflow as a function of time. 

Program RECESS, which also was developed by Rutledge 

(1998) and is included in the GW Toolbox, can be used to 

estimate K from a streamflow hydrograph during prolonged 

periods of negligible recharge. No changes were made to the 

RORA or RECESS algorithms for the GW Toolbox.

Each of the seven hydrograph-analysis methods is 

based on a number of simplifying assumptions that limit 

their applicability. These assumptions are described by many 

investigators, including Sloto and Crouse (1996), Rutledge 

(1998, 2000, 2007), and Healy (2010), and are discussed in the 

remainder of this section. Moreover, although the programs 

calculate daily and (or) monthly values of base flow, runoff, 

and recharge, the estimates are most reliable when averaged 

over longer time periods, such as years.

The methods should be applied to the analysis of 

streamflow hydrographs that reflect contributions from two 

sources: surface runoff in response to a precipitation event 

and groundwater discharge from a single aquifer. The methods 

are intended for application to basins dominated by diffuse 

areal groundwater recharge that is uniformly distributed over 

a basin, rather than basins in which focused groundwater 

recharge, such as occurs from losing stream reaches, is the 

dominant recharge pathway. All groundwater recharge within 

the basin is assumed to discharge to the receiving stream network except that amount that is evapotranspired directly from 

the groundwater system (sometimes referred to as riparian 

evapotranspiration). As noted by Rutledge (2007) and Healy 

(2010), the hydrograph-separation methods also are based 

on the assumption that groundwater discharge to streams is a 

continuous process, whereas the recession-curve displacement 

method is based on the assumption of episodic recharge in 

response to storms. Several studies cited by Rutledge (2007) 

and Healy (2010, p. 89), have found that recharge estimates 

determined from the recession-curve displacement method 

are generally greater than those estimated by the hydrographseparation methods, as implied by the assumption that some 

groundwater recharge discharges by riparian evapotranspiration instead of by discharge to the stream network.

The assumption that all groundwater recharge either 

discharges to the stream or by groundwater evapotranspiration 

implies that the surface-water and groundwater drainage areas 

to the streamgage are coincident. Moreover, it is assumed that 

there is no loss of groundwater to underlying regional groundwater-flow systems or to groundwater withdrawals.

A number of hydrologic processes and human activities 

that affect the flow and storage of water within a basin can 

obscure the contributions from surface runoff and groundwater 

discharge to a streamflow hydrograph. These include snowmelt runoff and drainage from large lakes and wetland areas. 

Streamflow regulation, such as occurs at reservoirs, by streamflow diversions, or by wastewater return flows, also changes 
4 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

the natural flow of a stream. The algorithms that are the basis 

for the methods available in the GW Toolbox cannot differentiate among the various causes of hydrograph fluctuations 

and may incorrectly identify snowmelt, reservoir releases, and 

so forth as groundwater discharge. Therefore, users should 

understand the hydrologic conditions in a basin to confirm the 

quantitative results of the hydrograph-analysis methods.

Rutledge (1998, 2000) provides guidance on the size 

and slope of basins to which the methods should be applied. 

He suggests that the methods should not be applied to basins 

having drainage areas smaller than 1 square mile (mi2

; about 

3 square kilometers [km2

]), so that the duration of surface 

runoff exceeds the minimum time increment of streamflow 

data (1 day). A maximum drainage area of about 500 mi2

(about 1,300 km2) also is recommended, but the upper limit on 

basin size will depend on the degree of uniformity of rainfall 

over the basin, the occurrence of multiple hydrogeologic areas 

within the basin, and the relative importance of bank-storage 

effects. Rutledge (2000) further notes that, in basins with 

extremely low relief, the time period of surface runoff may 

not be accurately determined by use of equation 1; the effect 

of low slope on the analysis is likely to be exacerbated as the 

drainage basin increases in size.

There has been some discussion in the literature concerning the various limitations of the RORA method for estimating 

groundwater recharge. Healy (2010, p. 89–90), for example, 

notes that “Few, if any, aquifers conform to all of the assumptions inherent in the derivation of the recession-curve displacement method.” Halford and Mayer (2000) and Halford (2008) 

question the use of equation 1 to determine the duration of 

surface runoff from a basin, the use of streamflow records to 

estimate an appropriate recession index (K), and the effects 

of hydrologic phenomena such as bank storage and snowmelt on the identification of recharge peaks in a streamflow 

record. Nevertheless, there have been many applications of 

the RORA method since the initial release of the program in 

1994 (Rutledge and Daniel, 1994) and these applications have 

resulted in recharge values that are consistent with other basinscale recharge-estimation methods. The reader is referred to 

Rutledge (2007) and Healy (2010, p. 89–90) for reviews and 

summaries of these several studies, and to Risser and others 

(2005) and Delin and others (2007) for detailed studies in 

which results of the RORA method were compared to other 

recharge-estimation methods in humid regions.

Figure 1 summarizes several of the assumptions and limitations of the hydrograph-analysis methods provided with the 

Groundwater or surface-water development 

(groundwater pumping, surface-water discharges or diversions, and so forth) Streamflow record

None

Relatively minor 

(small component of typical 

low-flow conditions)

Substantial 

(large component of typical 

low-flow conditions)

Complete and 

<1 year in length

Complete and 

≥1 in length

Streamflow conditions Hydrogeologic 

units

Size of watershed, 

in square miles

Gaining Losing Perennial Intermittent Single Multiple ≤1 >1 to ≤500 >500

Time scale of analysis period Basin slope, in percent

Less than monthly 

(daily, hourly, event) Monthly or seasonal Annual or longer <1 ≥1

EXPLANATION

Appropriate

Caution

Not appropriate

less than; >, greater than; ≤, less than or equal to; ≥, greater than or equal to).

Figure 1. Guidance on appropriateness of hydrograph-analysis methods provided with the Groundwater Toolbox (<, 
Getting Started 5

GW Toolbox as a guide for their use. In determining whether 

or not the methods are appropriate for a particular basin, the 

overall weight of the guidance provided in figure 1 should 

be considered. For example, a user could be fairly confident 

that the methods could be applied meaningfully to a gaining, perennial stream in a basin of about 100 mi2

 in drainage 

area, but should be very cautious applying the methods to an 

intermittent stream with a drainage area of 1,000 mi2 and very 

low slope. Because of the several assumptions and guidelines 

described above, the authors of this report agree with the 

conclusion reached by Healy (2010, p. 96) concerning application of the hydrograph-analysis methods “...that a complete 

evaluation of these assumptions may require substantially 

more effort than actual application of the methods.” When 

possible, the methods should be combined with other types 

of data to confirm the hydraulic interaction of groundwater/

surface-water systems, such as simultaneous measurements of 

groundwater levels and stream stages, seepage-meter measurements, seepage runs, and chemical and heat tracers (Stonestrom and Constantz, 2003; Rosenberry and LaBaugh, 2008; 

Eddy-Miller and others, 2012).

Example Basin

The methods provided with this initial version of the 

GW Toolbox are demonstrated with streamflow data collected 

at the streamgage on the Pawcatuck River at Wood River 

Junction, Rhode Island (USGS streamgage 01117500). The 

drainage area to the streamgage is 100 mi2. Approximately 

42 percent of the basin is underlain by stratified-drift glacial 

deposits, which constitute the primary aquifer. The remainder 

of the basin is underlain by bedrock or till-covered bedrock, 

which constitutes a secondary aquifer, in the upland areas 

(Cervione and others, 1993). The Pawcatuck River streamgage 

was selected because conditions at the site closely—but not 

exactly—meet the major assumptions of the hydrograph-separation and recharge-estimation methods. Specifically, recharge 

to the aquifer is areally diffuse and there is little water use or 

streamflow regulation in the basin (Bent and others, 2011). 

However, groundwater and surface-water drainage basins to 

the streamgage are not completely coincident because some 

groundwater outflow is known to occur across the northeastern boundary of the drainage basin and groundwater inflow 

is suspected to occur across the western boundary of the 

basin (Bent and others, 2011). Nevertheless, these rates of 

groundwater flow are small compared to the total flow at the 

streamgage and are unlikely to have a substantial effect on the 

results of the analysis. The period of analysis was the 30-year 

period extending from January 1, 1971, through December 

31, 2000. This period was chosen for consistency with other 

studies in the basin and because of the availability of meteorological data from a weather station within the basin for this 

time period.

Getting Started

The first steps in the use of the GW Toolbox for any 

analyses are to build a GW Toolbox project for the geographic 

area of interest, retrieve spatial and time-series datasets to populate the project, and review the time-series data. This section 

of the report provides instructions that can be used to complete 

these tasks. It is assumed that the GW Toolbox has already 

been installed on the user’s computer; information on how to 

download and install the program can be found at the Web site 

provided in the Preface to this report.

A number of text-formatting conventions are used 

throughout the report. Components of the GW Toolbox interface, such as dialog boxes, panels, or buttons, are referred to 

in quotes, such as the “Preview Map” panel. Specific menu 

options and tools are referred to by bold text, such as the 

“Open Project” option in the “File” menu, which is available 

on the main toolbar.

Building, Saving, and Reopening a Project

A GW Toolbox project is a collection of spatial and 

time-series data and analysis results for a specific geographic 

area. The project area can consist of a State, county, or HUC–8 

hydrologic cataloging unit. A cataloging unit is one of the divisions of the hydrologic unit code (HUC) classification system 

of river basins in the United States (Seaber and others, 1987). 

Each cataloging unit is given an 8-digit code; for example, the 

Pawcatuck River streamgage is within the Pawcatuck-Wood 

cataloging unit (01090005). (Cataloging units also are referred 

to as basins in this report.)

Double-clicking on the “USGS GW Toolbox” icon 

results in the two dialog boxes shown in figure 2: the “USGS 

GW Toolbox 1.0” and “Welcome to USGS GW Toolbox 1.0” 

dialog boxes. If the GW Toolbox has not previously been 

used, the user should select the “Build New Project” option. 

Selecting this option produces a map of the United States 

showing the boundaries of States and cataloging units (fig. 3). 

The user can then zoom into an area of interest with the “Pan” 

and “Zoom” tools.

Labels can be added to identify the cataloging units in a 

zoomed-in view. This is done by right-clicking on the Cataloging Units layer, which results in several menu options 

(fig. 4A); in this example, the user selects the “Label Setup” 

option, which opens the “Label style” dialog box (fig. 4B). 

The user double-clicks on the “CU” field and then selects 

the “Apply” check box. A label-generation dialog box then 

appears (not shown on fig. 4) that allows the user to select the 

position and orientation of the labels. Once these are selected, 

the cataloging-unit labels are added to the map (fig. 4C).

The “Select” tool can then be used to select the cataloging unit of interest (01090005, fig. 5A). The user selects the 

“Build” check box from the “Build New USGS GW Toolbox 

1.0 Project” dialog box (fig. 3). At this point, the GW Toolbox 

will prompt the user with a “Save new project as...” dialog 
6 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Figure 2. The “Welcome to the USGS GW Toolbox 1.0” and “USGS GW Toolbox 1.0” dialog boxes that appear on first use of the 

Groundwater Toolbox.

box to select a directory name and location in which to save 

the project. The default location in which to save the project 

is the C:\USGS-GWToolbox\data directory, and a subdirectory is created for the project with the HUC–8 code as the 

default directory name (which in this example is 01090005). 

Project information is stored in a MapWindow project file in 

the project directory (.mwprj file) with the HUC–8 code in the 

default file name (01090005.mwprj). If multiple projects have 

been created for the HUC–8 area, a sequence number will be 

appended to the subdirectory and project names (for example, 

01090005–2). A “Projection Properties” dialog box will appear 

and the user can select the desired projection for the project 

data layers [the default “Albers Equal-Area (Conterminous 

U.S.)” option was selected for this example]. The project 

then will be populated with a set of standard data layers. To 

view all of the layers that are active, it is helpful to close the 

“Preview Map” panel by clicking on the “X” on the top right 

of the “Preview Map” panel or by toggling off the check-mark 

in the “Preview Map” section from the “Panels” option in the 

“View” menu. The resulting map and data layers are shown in 

figure 5B.

It should be noted that the “Welcome to USGS GW 

Toolbox 1.0” dialog box can be reopened at any time using 

the “Welcome Screen” option from the “Help” menu. Also, 

as an alternative to the approach described in the previous 

paragraphs, a new project can be built by selecting the “New” 

option from the “File” menu. An existing project can be 

reopened from the “Welcome to USGS GW Toolbox 1.0” dialog box or with the “Open Project” or “Open GW Toolbox 

Project” options from the “File” menu.
Getting Started 7

Figure 3. The initial map with data layers active that show states and hydrologic cataloging units. “Pan,” “Zoom In,” “Zoom Out,” and 

“Select” tools are identified.

Time-Series Data Discovery and Retrieval

When a GW Toolbox project is created, a large cache 

of map layers is downloaded and stored in the project directory. A subset of these layers, which is shown in the “Legend” 

panel (fig. 5B), is made available as part of the initial build of 

the project. Data-collection sites for several types of observed 

data are shown by default on the project map. These are sites 

at which a minimum of 10 data values of daily streamflow 

(discharge), daily groundwater levels, and periodic groundwater levels are available through the NWIS database. The sites 

with periodic groundwater-level data are not actively displayed on the map because they tend to be numerous and are 

not of primary interest for the tools described in this report. 

Their locations can be seen by toggling the check-box next to 

the layer identifier to the “on” position.

The first step in data retrieval is to select specific data 

sites from the project map for which data will be downloaded 
8 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

A

B

Figure 4. The method for adding labels to a map layer. In this example, cataloging-unit numbers are added to the Cataloging Units 

layer. A, Options available by right-clicking on the selected layer, “Cataloging Units.” B, “Label-style” dialog box that appears after 

selecting the “Label Setup” option.
Getting Started 9

C

Figure 4. The method for adding labels to a map layer. In this example, cataloging-unit numbers are added to the Cataloging Units 

layer. C, The resulting map, with cataloging-unit labels added.—Continued

from NWIS. For this example, streamflow data will be 

retrieved because they are needed for the base-flow and 

recharge analyses. The discharge-locations data layer shown 

on the map indicates that there are several streamgages within 

and near the Pawcatuck-Wood Basin (fig. 5B). At this point, 

it is convenient to either add the streamgage numbers using 

the “Label Setup” option, as described previously (fig. 4), or 

to import a data layer of the drainage-basin boundary for the 

streamgage of interest, if one is available. For this example, 

a shapefile of the drainage basin for the Pawcatuck River at 

Wood River Junction streamgage (01117500) was imported 

by use of the “Add Layer” option from the “Layer” menu. 

Streamgage identifiers also were added to the map (fig. 6). To 

select a streamgage, the user clicks on the data layer of interest 

(“NWIS Daily Discharge Stations”; fig. 6, step 1) and then 

clicks on the “Select” tool (fig. 6, step 2). The user can then 

select the streamgage of interest by clicking on it (fig. 6, step 

3). Alternatively, one or more streamgages of interest can be 

selected by drawing a box around them with the “Select” tool 

or by using the “Ctrl” key on the keyboard and “Select” tool 

to select multiple stations for download. The selected stations 

will then appear grayish yellow on the map.

After a site is selected, the “Download Data” option 

from the “File” menu can be used to retrieve data from NWIS 

for the station (fig. 7A). As shown on figure 7A, there are several types of data that can be downloaded, but for this example 

the “Daily Discharge” button within the “Data Values from US 

Geological Survey National Water Information System” panel 

already has been selected because of the steps completed in 

the previous paragraph. The user then clicks the “Download” 

button and data for the selected site are retrieved (there is a 

pause as the data are retrieved). The user is then informed that 

data for the selected site have been retrieved and saved in a 

file that uses the USGS NWIS RDB (relational database) timeseries format (fig. 7B). The daily discharge data are reported 

in units of cubic feet per second (USGS NWIS parameter code 

00060). The data are now part of the project and available for 

use in the GW Toolbox.

Similar steps were taken to retrieve daily groundwater-level data for observation well RI–EXW 554 (USGS 
10 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

A

B

Figure 5. Selected steps to build a Groundwater Toolbox project. A, The “Select” tool is used to select the cataloging unit (basin) of 

interest, which is then shaded yellow. B, The resulting map, which includes daily streamflow (discharge) and groundwater-level datacollection locations for the basin of interest and surrounding area.
Getting Started 11

observation well 413252071323601), which is within the 

Pawcatuck River Basin. Groundwater-level data are stored in 

NWIS in a number of formats; for example, depth to water 

level in feet below land surface (parameter code 72019) or 

elevation of water (parameter code 72020). The GW Toolbox 

converts all data formats to depth to water level, in feet below 

land surface.

As part of the data-discovery process, it can be advantageous to add other data layers to complement the default data 

layers shown on the project map. This can be done by toggling 

inactive data layers shown on the “Legend” (such as “Major 

Roads”) to active (by clicking on the check boxes next to each 

data layer) or by retrieving data layers that have been preprocessed for the GW Toolbox and are available for download. 

These data layers are listed on the “Download Data” dialog 

box (fig. 7A). After the user selects a particular data layer and 

then clicks on the “Download” button, there may be a pause 

while the requested data files are downloaded and projected, 

merged, and clipped as needed to the download region (which, 

in this case, is hydrologic unit 01090005). When the download is complete, the user will be returned to the GW Toolbox 

interface, and the added layers will appear on the explanation 

and map.

The user also can adjust the minimum number of measurement values required at an observation site for the site to 

be shown on the project map. This is done by adjusting the 

value specified in “Min Count” on the “Download Data” dialog box. The default value for “Min Count” is 10 measurement 

values, but this value can be modified. After the value of “Min 

Count” is adjusted for a selected data type (“Daily Groundwater,” “Periodic Groundwater,” and so forth), the user clicks on 

the “Download” button, which causes the map to be updated 

with sites that meet the minimum specified count.

Summarizing, Listing, and Graphing Time-Series 

Data

It is important to carefully screen the streamflow data 

prior to a hydrograph analysis to identify suspect data and 

to confirm that the data are consistent with the underlying 

assumptions of the analysis methods (Sloto and Crouse, 1996). 

The GW Toolbox provides three tools that are useful in this 

regard: the “Data Tree,” “List,” and “Graph” tools under 

the “Analysis” menu option. The “Data Tree” tool provides 

a listing of many of the attributes and summary statistics for 

the dataset, whereas the “List” tool provides a listing of the 

complete dataset. Each of the three tools is explained in detail 

in online documentation under the “Help” menu option.

Selection of any of the three tools brings up either the 

“Select Data” or “Select Data to Graph” dialog boxes (fig. 

8A), which allow the user to select one or more time-series 

Step 1

Step 2

Step 3

Figure 6. Steps used to select one or more data sites for which data can be retrieved from the U.S. 

Geological Survey National Water Information System for use in the Groundwater Toolbox. In this 

example, the streamgage on the Pawcatuck River at Wood River Junction, Rhode Island, is selected 

(site 01117500). Steps are discussed in the text. Part of the drainage basin for the streamgage is 

shown in red.
12 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

datasets available for the project. For this example, the 

“Graph” option will be illustrated. The top section of the 

“Select Data to Graph” dialog box is titled “Select Attribute 

Values to Filter Available Data” and contains dropdown 

lists with time-series attributes. The number and content of 

these lists can be managed through the “Attributes” menu. 

The default attributes that are shown with the GW Toolbox 

are “Scenario,” “Location,” and “Constituent,” but these 

default selections can be changed by the user. Attributes 

are shown in figure 8 for two observed datasets that were 

retrieved in previous steps: the streamflow data for streamgage 

01117500 and the groundwater-level data for observation well 

413252071323601. When the user selects an attribute from 

any of the lists, the “Matching Data” section is populated with 

the datasets that meet the attribute(s) selected. In the example 

shown on figure 8A, “Streamflow” has been selected from 

the “Constituent” attribute list (step 1), which brings up the 

only matching dataset at streamgage 01117500. The user then 

selects this streamgage (step 2) and the selected time series 

shows up in the “Selected Data” section of the dialog box 

(step 3). Note that the complete date range of the streamflow 

dataset is shown in the “Dates to Include” section of the dialog 

box; the “Start” and “End” dates of the graph can be modified 

in the white fields. The user then clicks on “Ok” to bring up 

the graphing options (fig. 8B). For this example, both the timeseries and flow-duration plots are selected.

The time-series graph (and list of the data, which is not 

shown) indicate that data are complete throughout the period 

of record (December 7, 1940, through November 5, 2014; 

fig. 9A); however, data for the period October 1 through 

November 5, 2014, are provisional data that have not yet been 

approved for publication by the USGS. These provisional data 

A

B

Figure 7. Dialog boxes used to retrieve data from the 

U.S. Geological Survey National Water Information 

System and other sources for use in the Groundwater 

Toolbox. A, The “Download Data” dialog box used to 

retrieve streamflow (discharge) data. B, The dialog box 

indicating that the data for the site of interest (01117500) 

have been retrieved and are available for use.
Getting Started 13

are shown in red on the graph; they are difficult to identify 

on the graph because of the very short period of provisional 

data for this example. It is important to note that many of the 

basic computations in the GW Toolbox use data—including 

provisional data—for the entire period of record as the default 

option for the computations (such as determination of mean 

streamflow). The user can remove the provisional data from 

the computations by toggling off the “Include Provisional 

Data” button on the “Select Data” dialog box, or by changing 

the end date in the “Dates to Include” section of the “Select 

Data” dialog box (fig. 8A). The user can determine the exact 

date range of the provisional data with the “List” tool (under 

the “Analysis” menu option). Within the “Time Series List” 

dialog box, the user selects the “Value Attributes” option 

under the “View” option. This action adds several columns 

to the “Time Series List” dialog box, including the “A” (for 

approved) and “P” (for provisional) columns. Data identified 

as “True” (or “Tr+”) in the “P” column are provisional.

The time-series graph and flow-duration plot (fig. 9) 

indicate that streamflow at the streamgage is characteristic of 

streams that have groundwater discharge (base flow) as a primary source of water. These characteristics include perennial 

flow throughout the measurement period (with a minimum 

value of 15 cubic feet per second [ft3/s] on October 11, 1947) 

and a flow exceedance that is greater than about 22 ft3

/s more 

than 99.8 percent of the time (fig. 9B).

Meteorological Data

The GW Toolbox contains several types of meteorological data that can be useful in the analysis of hydrologic 

conditions and water budgets for a basin. The primary source 

of meteorological data is the BASINS 4.0 meteorological 

database, which is derived from data acquired from the NOAA 

NCDC. The database contains data for more than 16,000 

weather stations through 2009, although not all stations are 

still active and most stations contain only a subset of the seven 

meteorological data types that are provided: precipitation, air 

temperature, wind speed, cloud cover, potential evapotranspiration, solar radiation, and dewpoint temperature. Typically, 

for a high-quality station, data extend from 1948 through 

2005. A full description of the datasets that are available and 

how they were processed for the BASINS 4.0 database is 

provided in the online documentation. Short descriptions of 

the datasets used for the analyses described in this report are 

provided below; these datasets are a subset of the full suite of 

datasets available in the GW Toolbox.

A shapefile of the locations of weather stations in a project area can be downloaded by use of the “Download Data” 

option by selecting the “Hydrologic Units” option in the 

“Region to Download” panel and the “Met Stations” button 

under the “BASINS” section of the dialog box. Once the locations are shown on the map, the user can select the weather 

stations of interest with the “Select” tool and then retrieve the 

data for the selected stations by use of the “Download Data” 

dialog box (by selecting the “Hydrologic Units” option in the 

“Region to Download” panel and the “Met Data” button under 

“BASINS” section of the dialog box). A “Met Data Processing 

Options” dialog box will appear, and the user can select “Add 

individual files (one per station) to project” button, which then 

makes the data available for use.

For this example, data from the Kingston, R.I., weather 

station (station RI374266) were downloaded because the station is within the example basin. The download resulted in the 

retrieval of six datasets, PRCP, PREC, TMAX, TMIN, ATEM, 

and PEVT, which are described as follows:

• PRCP: Observed daily precipitation (in inches) from 

the “Cooperative Summary of the Day” NCDC source,

• PREC: Computed hourly precipitation (in inches) from 

the “Hourly Precipitation disaggregated from Daily” 

NCDC source,

• TMAX, TMIN: Observed daily maximum and minimum air temperatures, respectively, (in degrees Fahrenheit) from the “Cooperative Summary of the Day” 

NCDC source,

• ATEM: Computed hourly air temperature (in degrees 

Fahrenheit),

• PEVT: Computed hourly potential evapotranspiration 

(in inches) by use of the methods described in Hamon 

(1961).

Summary statistics provided in the “Data Tree” option 

from the “Analysis” menu indicate the following long-term 

(1971–2000) values for precipitation, air temperature, and 

potential evapotranspiration at the Kingston weather station: mean daily precipitation (PRCP), 0.14188 inch (in.) (or 

51.82 inches per year [in/yr]); mean daily air temperature 

(ATEM), 49.59 degrees Fahrenheit (°F; 9.77 degrees Celsius 

[°C]); mean daily maximum air temperature (TMAX), 60.73 

°F (15.96 °C); mean daily minimum air temperature (TMIN), 

39.38 °F (4.10 °C); and mean hourly potential evapotranspiration (PEVT) of 0.0029034 in. (or 25.45 in/yr).

The GW Toolbox also provides a second source of 

precipitation data, which is available from the USGS NWIS 

database (USGS NWIS parameter code 00045). The locations 

of sites with precipitation data from NWIS can be added to a 

project map using the “Download Data” option in the “File” 

menu and by selecting “Precipitation” button under the “Station Locations from US Geological Survey National Water 

Information System” section. USGS precipitation collection 

sites are sparsely distributed and, in fact, none were found near 

the Pawcatuck River Basin.
14 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

A

Step 1

Step 2

Step 3

B

Figure 8. Dialog boxes used to select and graph data for use in the Groundwater Toolbox. A, The “Select Data 

to Graph” dialog box for selecting time-series data to graph. B, The “Choose Graphs to Create” dialog box to 

select graph types. Steps are described in the text.
Getting Started 15

A

B

Figure 9. Graphs of streamflow data at the example streamgage on the Pawcatuck River at Wood 

River Junction, Rhode Island, for use in the Groundwater Toolbox. A, Streamflow hydrograph. B, Flowduration curve.
16 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Base-Flow and Runoff Components of 

Streamflow: Hydrograph Separation

The base-flow and runoff components of streamflow are 

determined by use of the “USGS Base-Flow Separation” 

option in the “Analysis” menu. This menu option brings up 

the dialog box “Select Daily Streamflow for Analysis,” which 

is similar to the “Select Data to Graph” dialog box described 

previously and shown in figure 8A. The user selects a single 

streamflow dataset from the list of available datasets for 

analysis; in this example, data for site 01117500 were selected. 

Selection of a dataset will result in the “Base-Flow Separation” dialog box shown in figure 10A.

The user selects one or more of the six available hydrograph-separation methods. If the BFI–Standard or BFI–Modified methods are selected, the GW Toolbox will populate the 

“BFI Parameters” panel of the dialog box with default values: 

a partition length (N) of 5 days, a turning point test factor (f) of 

0.9, and (or) a recession constant (K’) of 0.97915. Also, for the 

BFI options, the user can specify that results will be written 

to the original BFI output files by “Calendar Year” or “Water 

Year.” This option is only available for the BFI programs. If 

the drainage area to the streamgage is available as an attribute 

with the NWIS streamflow data, the GW Toolbox will automatically populate the panel with that value, as shown for the 

Pawcatuck River streamgage (100 mi2

); however, the user can 

modify the drainage-area value as desired.

The user then defines the dates for analysis. The “Examine Data” button can be used to determine which months 

have complete records (fig. 10B); the “Examine Data” utility 

was originally released with the PART software distribution 

as the “screen” program. If “Water Year” has been selected 

for the BFI options, the user should specify that the analysis 

begins on October 1 of the year preceding the first water year 

of interest and ends on September 30 of the last water year of 

interest. For example, if the user is interested in water years 

1938 through 1945, “Analysis Dates” of 1937/10/01 through 

1945/09/30 would be specified. Complete years of record 

are available for the Pawcatuck River streamgage between 

1941 and 2012, but only 1971 through 2000 will be analyzed 

in the example (fig. 10A), for consistency with the available 

meteorological data.

Many output files will be created by running the hydrograph-separation programs, and these should be saved in a 

directory that is specified in the “Text Output” section of the 

dialog box. Each file will have a base name or prefix that also 

is specified in this section of the dialog box. After specifying the output directory, the user clicks the “Run Base-Flow 

Separation Program(s)” button to run the programs. If all six 

methods have been selected, there can be a pause of several 

seconds while the programs complete their calculations. When 

completed, a dialog box will appear that says “Base flow 

output completed.”

The user can now examine the output files or graph the 

results of the programs. Inspection of the several files in the 

output directory indicates that the GW Toolbox creates not 

only the original output files generated by each of the individual programs but also several comma-delimited (.csv) output 

files that include results of all methods selected for analysis. 

These comma-delimited files include output at daily, monthly, 

and annual (by calendar year) time intervals for the period of 

analysis; there also is a duration.csv file that gives the percentage of time that each streamflow, base-flow, or surfacerunoff value was equaled or exceeded. Unlike some previous 

versions of the original programs, the PART, HYSEP, and 

BFI methods implemented in the GW Toolbox can be used to 

calculate base flow and runoff for periods of record that span 

only part of a full calendar year; however, if the dates defined 

for the analysis span only part of a particular month or calendar year, results in the monthly.csv and yearly.csv files will not 

include the partial months or years.

Several graphing options also are available in the 

“Display Graph” panel to review the hydrograph-separation 

results. A particularly useful option is “Flow Duration,” 

which creates flow-duration graphs of the observed streamflow and calculated base-flow and runoff results for each 

method. The flow-duration graphs for the Pawcatuck River 

streamgage indicate that base flow is the predominant source 

of water to the river at nearly all flow-duration levels. The 

graphs also demonstrate a fair amount of variability in the 

hydrograph-separation results, particularly at the highest 

streamflows (fig. 11). The PART method generates the highest 

base-flow estimates throughout the range of flows, whereas 

the two BFI methods generate the lowest estimates. These 

general results, however, do not represent all streamgages; for 

example, several initial tests with the GW Toolbox with data 

from streamgages across the United States indicated that the 

methods that yield the highest and lowest values of base flow 

varied among streamgages.

Average annual rates of streamflow, base flow, and runoff 

can be calculated with a spreadsheet program from results 

listed in the yearly.csv file. For the 30-year period evaluated 

here, the average-annual rate of streamflow at the Pawcatuck 

River streamgage was 204 ft3/s, or 27.7 in. over the basin. 

Calculated rates of average annual base flow for the 30-year 

period range from minimums of 20.8 and 21.0 in., calculated by the two BFI methods, to a maximum of 24.9 in., 

calculated by the PART method; the mean of the six average 

annual values is 22.8 in. Average annual runoff ranges from 

a minimum of 2.8 in. for the PART method to 6.7 and 6.9 in. 

for the two BFI methods, with a mean of the six values of 

4.9 in. The value of base flow at the streamgage calculated 

by the PART method is close to that reported by Bent and 

others (2011) for the 63-year period from 1942 through 2004 

(23.9 in.), which was determined by use of the original PART 

program (Rutledge, 1998).
Groundwater Recharge 17

A B

Figure 10. Dialog boxes used in the base-flow separation analysis component of the Groundwater Toolbox. A, The “Base-Flow 

Separation” dialog box. B, Part of the “Data Summary” dialog box, which shows months having complete (dots) or incomplete 

(Xs) records.

Groundwater Recharge

Groundwater recharge is calculated by the RORA 

ecession-curve displacement method (Rutledge, 1998). As 

escribed previously, the RORA program requires that a 

ecession index be specified for the basin. This index can be 

etermined independently from the GW Toolbox or can be 

stimated by use of the RECESS program (Rutledge, 1998), 

hich is included in the GW Toolbox. The RECESS and 

ORA methods are invoked with the “USGS RECESS” and 

USGS RORA” options under the “Analysis” menu. Each 

f these menu options brings up the dialog box “Select Daily 

treamflow for Analysis,” which is similar to the “Select 

ata to Graph” dialog box described previously and shown in

gure 8A. The user selects a single streamflow dataset from 

he list of available datasets for analysis; in this example, data

or streamgage 01117500 were selected. The sections below 

escribe details for each analysis method.

r

d

r

d

e

w

R

“

o

S

D

fi

t

f

d

RECESS Program To Estimate Basin Recession 

Index

The RECESS program can be used to estimate a median 

value of the recession index (K), which is needed by the 

RORA program, as well as to perform several other functions 

that are described in detail by Rutledge (1998). The user is 

encouraged to read the sections of Rutledge (1998) concerning RECESS; the description here is intended to illustrate how 

RECESS has been implemented in the GW Toolbox. Selection of a streamflow dataset in “Select Daily Streamflow for 

Analysis” dialog box, accessed through the “USGS RECESS” 

option of the “Analysis” menu, will bring up the “USGS 

RECESS” dialog box shown in figure 12A.

The first step is to select the dates for which streamflow 

recessions will be analyzed. These dates need not coincide 

with the time period used in the hydrograph-separation or 

recharge analyses, although, for this example, coincident dates 

are specified (fig. 12A, “Analysis Dates” entries). The user 

also specifies an output directory for the RECESS results.
18 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Figure 11. The flow-duration curves for 

measured streamflow and estimated base 

flow and runoff for the Pawcatuck River 

at Wood River Junction, Rhode Island, 

from 1971 through 2000 using the six 

hydrograph-separation methods available 

in the Groundwater Toolbox.

The user then specifies the “Months and Seasons” for 

which recession periods will be analyzed; the program will 

select only those recession periods that begin in the selected 

months. For this analysis, only the four winter months from 

December through March were selected for analysis, because 

it is assumed that riparian evapotranspiration is negligible during these months. Riparian evapotranspiration can create nonlinearity in a recession graph of the logarithm of streamflow 

as a function of time. The “Winter” radio button also has been 

selected to coincide with the four selected months; selection 

of a season is used only to insert a text entry in the output files 

and does not affect the results.

Finally, the user specifies the minimum number of 

days required for detection of a recession period. Rutledge 

(1998) notes that if the number of days selected is too large 

(too restrictive), few recession periods will be detected in 

the record; however, if the number is too small, then many 

recession periods will be detected and some may need to be 

omitted, because they would not span a recession period long 

enough for the user to derive an accurate value of K. Rutledge 

(1998) suggests a minimum of 10 to 20 days when analyzing 

a record of 40 years. Values of 10 and 15 days were tested for 

this example; a total of 61 recession periods were identified for 

the streamflow record from 1971 through 2000 for the 10-day 

minimum requirement and only 17 recession periods were 

detected for the 15-day minimum requirement. Results for the 

15-day requirement are illustrated here.

The user then clicks the “Find Peaks” button (fig. 

12A), which brings up the list of recession periods that fit 

the minimum requirement of days of recession following a 

streamflow peak (fig. 12B). For this example, 17 recession 

periods are listed, the first having had a peak streamflow on 

January 7, 1971. Clicking on the date (“1971/ 1/ 7” in this 

example) brings up a list of the streamflows during this recession period, beginning one day after the peak (January 8) and 

continuing for 15 days (through January 22) until the next 

increase in streamflow on January 23. A semilogarithmic plot 

of the streamflow data is shown on the right of the dialog box.

The user can now either “Analyze” the streamflow data 

for the full recession period or eliminate some of the data if 

any part of the recession graph looks nonlinear. For this example, both approaches are illustrated. First, by clicking “Analyze” with all data selected, a recession index (“DAYS/LOG 

CYCLE”) of 38.432305 is calculated by RECESS (fig. 12C). 

In the second approach, the user may limit the analysis to 

periods of the recession that are nearly linear on the semilogarithmic plot. Rorabaugh (1964) and Rutledge (1998) note that 

the slope of the semilogarithmic recession graph, which is 

equal to the value of K, should be linear after a “critical time” 

(t

c ). This critical time, which is a function of the properties of 

the groundwater system (Rorabaugh, 1964; Rutledge, 1998), 

should be a constant value that is independent of any particular recession period selected for analysis. In practice, however, 

the apparent value of t

c

 may vary from one recession period 

to another. For example, for the recession period January 

8–22, 1971, the recession graph indicates there is a period of 

slight nonlinearity during approximately the first 4 days of 

recession. The user might conclude, therefore, that t

c

 equals 
Groundwater Recharge 19

A

B

C

Figure 12. Dialog boxes used for the RECESS component of the Groundwater Toolbox. A, Initial dialog box to specify the dates, months, 

and seasons for analysis, the minimum number of days required for detection of a recession period, and the output directory. B, Dialog 

box showing recession periods that meet the minimum-day requirement (left side of dialog box), the first recession period beginning 

after a peak streamflow on January 7, 1971 (middle part of dialog box), and a semilogarithmic graph of the first recession period (right 

side of dialog box). C, Dialog box showing the estimated recession index of 38.432305 days per log cycle for full period of recession from 

January 8 through January 22, 1971. 
20 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

D

E

Figure 12. Dialog boxes used for the RECESS component of the Groundwater Toolbox. D, The “Choose Recession Limb Duration” 

dialog box allows the user to refine the days that will be used to calculate the recession index. E, Dialog box showing a summary, the 

median value, and a scatter plot of the recession indices calculated by RECESS for the example basin.—Continued

4 days for this particular recession period, and then select the 

period January 12–20, 1971, for analysis. This is done by first 

clicking on the row of data identified as day 5 after the peak 

streamflow (fig. 12B), which brings up the “Choose Recession 

Limb Duration” dialog box (fig. 12D). The user then selects 

“5” as the “First Day” of analysis. This action restricts the 

number of days that will be analyzed to days 5 through 15. 

The data-reduction process can be repeated to eliminate data 

after day 13 that appear to diverge from the straight-line segment of the recession. This is done by selecting the row of 

data identified as day 13 after the peak streamflow and then 

selecting “13” as the “Last Day” of analysis in the “Choose 

Recession Limb Duration” dialog box; this further restricts 

the number of days that will be analyzed to days 5 through 13. 

With these days selected, a slightly higher recession index of 

38.713915 was calculated for this 9-day period (not shown on 

figure 12). Selecting the check box next to the start date of the 

recession period (that is, to the left of “1971/ 1/ 7”) will save 

the recession index for later use (fig. 12E).

After analyzing several periods of recession using the 

same process, the user can then click the “Summary” button to 

summarize the values of all recession indices calculated during the analysis (fig. 12E). For this example, varying lengths 

of recession periods were analyzed for 13 recessions selected 

during the record for 1971 through 2000. For these 13 recession periods, the first day of analysis ranged from 5 to 7; that 

is, t

c

 was determined to range from 4 to 6 for the 13 recession periods. In most cases, this number of recession periods 

would be too few for a formal analysis, but is sufficient for 

this example. Rutledge (1998) suggests that 20 to 30 periods 

of recession might be used for a typical analysis; Rutledge and 

Mesko (1996, table 2) analyzed from 6 to 49 recession periods 

for 157 streamflow records in a study of streamflow recession and base flow in the Appalachian Valley and Ridge, Blue 

Ridge, and Piedmont Physiographic Provinces that extend 

from Pennsylvania southward into Alabama. The summary 

listing of the 13 calculated recession indices is given in the 

middle panel of the dialog box and a graph of the indices (by 

date) is given on the right side of the dialog box. The median 

recession index calculated for the 13 recession periods is 

65.93 days per log cycle, but, as seen in figure 12E, there is a 

range of calculated values distributed about this median value 

that extends from a minimum of 37.323 days per log cycle to 

a maximum of 104.323 days per log cycle (fig. 12E). The user 

can remove one or more of the calculated recession indices 

from the summary listing by unchecking their corresponding 

dates of peak streamflow on the left side of the dialog box 

and clicking again on the “Summary” button. For example, if 

the recession index of 104.323 days per log cycle calculated 

for the period following the “1978/2/11” peak is removed, a 

new, slightly lower, median recession index of 65.59 days per 

log cycle is calculated for the remaining 12 recession periods. 
Water Budgets for the Example Basin 21

Each time the “Summary” button is clicked, the median value 

of the recession index will be written to the index.txt file in the 

output directory.

RORA Program To Estimate Groundwater 

Recharge

The RORA program is accessed through the “USGS 

RORA” option from the “Analysis” menu. As on the “BaseFlow Separation” dialog box, the user selects a period of 

analysis and an output directory (folder) and base name (prefix) for the output file on the “USGS RORA” dialog box (fig. 

13). The drainage area of the basin is automatically loaded 

into the RORA dialog box if it is available from the downloaded NWIS data. Also, if the RECESS program has been run 

to determine a recession index for the basin in the same GW 

Toolbox session, then the final estimate of the median recession index will be loaded automatically into the RORA dialog 

box. Alternatively, the “Browse Recession Index” button can 

be used to navigate to a directory in which the index.txt file, 

which holds values of the median recession indices calculated 

by the RECESS program, can be opened and a specific recession index identified for use in the RORA analysis. Values of 

the drainage area and recession index that have been loaded 

automatically can be changed by the user if desired.

The program also automatically calculates the value of 

the antecedent-recession requirement, which is equivalent to 

the duration of surface runoff for the basin (Nsr, in days; see 

the “Base-flow, Runoff, and Recharge Estimation Methods and 

Applicability” section). The default value of the antecedentrecession requirement can be modified by the user. If the 

streamflow record includes zeros, the program will replace the 

zeros with the default value of 0.01 ft3/s (which can be modified by the user), so that extrapolation done by the program on 

the log scale is possible.

After each section of the dialog box has been completed, 

the user can click on either the “Plot Monthly Recharge” or 

the “Write ASCII Outputs” button to run the RORA program. 

Clicking on the latter button creates several files in the output 

directory; these files include those generated by the original 

RORA program (Rutledge, 1998) and two comma-delimited 

data files that hold the monthly (roramon.csv) and annual (roraAnn.csv) recharge rates calculated by RORA (in inches over 

the basin). An average annual recharge rate for the 30-year 

period evaluated here for the example basin was calculated to 

be 26.6 in. with a spreadsheet program from the results listed 

in the Pawcatuck_roraAnn.csv file. This value of basin-wide 

recharge is similar to that calculated by Bent and others (2011) 

by use of streamflow data at the streamgage for the 63-year 

period from 1942 through 2004 (25.9 in.) determined by use of 

the original RECESS and RORA programs (Rutledge, 1998). 

The monthly recharge rates calculated for the analysis period 

are shown in figure 14.

Although the RORA method is based on the assumption 

that the value of the recession index is constant throughout a 

basin, in practice, as the example showed, there can be substantial variability in the values calculated from a streamflow 

record. Rutledge (1998), however, has shown that the results 

of RORA are only slightly sensitive to variation in the value 

specified for the index. This sensitivity is illustrated by varying the value of the recession index estimated for the analysis 

of the Pawcatuck River streamflow data by ±50 percent. In 

the first sensitivity test, the recession index was decreased 

to a value of 32.80 days per log cycle. The resulting average 

annual recharge rate for the 30-year period increased by 0.40 

in. (from 26.64 to 27.04 in.), an increase of only 1.5 percent 

above the base value. In the second test, the recession index 

was increased to a value of 98.39 days per log cycle. The 

resulting average annual recharge rate decreased by 0.67 in. 

(to 25.97 in.), a decrease of 2.5 percent below the base value. 

These results indicate that, although the value of the recession 

index may be quite variable—as was the case for the Pawcatuck River Basin estimates—the user should not spend an 

inordinate amount of time attempting to refine the estimates of 

the recession index calculated with the RECESS program.

Water Budgets for the Example Basin

Water budgets are an important tool for evaluating and 

managing the availability and sustainability of water supplies 

for a particular basin (Healy and others, 2007). As demonstrated in previous sections of the report, the GW Toolbox 

provides methods to quantify several components of a basinwide water budget, such as base flow, runoff, and groundwater 

recharge. This section illustrates how the GW Toolbox can be 

used to quantify several additional water-budget components 

for a basin, and then demonstrates how these individual waterbudget components can be used to determine an estimate of 

basinwide, long-term average water budgets. The approach 

taken here is similar in several ways to that described by 

Rutledge and Mesko (1996) for water budgets determined 

for basins in the Appalachian Valley and Ridge, Blue Ridge, 

and Piedmont Physiographic Provinces. These methods are 

demonstrated here for the Pawcatuck River Basin at the Wood 

River Junction streamgage for the 30-year period from 1971 

through 2000. A summary of the methods used to estimate 

each water-budget component, as well as the values estimated, 

is provided in table 1.

A water budget accounts for all of the inflows, outflows, 

and changes in storage of water throughout a basin. For the 

Pawcatuck River Basin, components of the water budget that 

may need to be quantified are precipitation, streamflow into 

and out of the basin, groundwater underflow into and out of 

the basin, water-use transfers into and out of the basin, evapotranspiration, and changes in water storage throughout the 

basin. The water-budget equation is written as

SFi GW P + + i WU   SFo o GW WU

+ i = + + o

  + ET + ∆S , (3) AA A   A A A
22 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Figure 13. The “USGS RORA” dialog box to estimate 

groundwater recharge by use of the RORA program in 

the Groundwater Toolbox.

where

P is the average annual rate of precipitation 

(length over time),

SFi

 and SFo are the average annual rates of streamflow 

into and out of the basin (cubic length over 

time), respectively,

A is the drainage area of the basin (square 

length),

GWi and GWo are the average annual rates of groundwater 

underflow into and out of the basin (cubic 

length over time), respectively,

WUi

 and WUo are the average annual rates of water transfers 

into and out of the basin (cubic length over 

time), respectively,

ET is the average annual rate of 

evapotranspiration from the basin (length 

over time), and

	 ΔS	 is the total storage change in the basin (length 

over time).

Equation 3 is a modified version of water-balance equations presented by Healy and others (2007) and Sanford and 

others (2012), and reflects the authors’ conceptual model of 

the important components of the hydrologic system at the 

scale of the watershed (100 mi2

) over the 30-year analysis 

period. The terms on the left side of the equation are the flows 

into the basin, whereas those on the right are the flows out of 

the basin (in parentheses) and the change in water storage in 

the basin. It should be acknowledged that there are inherent 

uncertainties in the estimates of each water-budget term that 

reflect the accuracy by which each term can be estimated from 

the methods available in the GW Toolbox. These uncertainties 

have not been quantified, but should be recognized as being 

present. Thus, the right- and left-hand sides of equation 3 

must be balanced by an error term, which is not shown but is 

implicit in the equation.

Several of the terms in equation 3 are either known to 

equal zero or can be assumed to equal zero for the example of 

the Pawcatuck River Basin. First, there is no streamflow into 

the basin, so that SFi

 is zero. Second, although it is known that 

there are small amounts of groundwater flow into and out of 

the basin (Bent and others, 2011), it is assumed that the surface-water and groundwater drainage areas to the streamgage 

are the same, so that the two groundwater-underflow terms, 

GWi

 and GWo , are zero. Third, Bent and others (2011) indicate 

that there are relatively few internal withdrawals from either 

the groundwater or surface-water systems within the basin or 

interbasin transfers to or from adjacent basins; therefore, the 

two water-use transfer terms into and out of the basin, WUi

and WUo

, can be assumed to be zero. Finally, it is assumed that 

during the 30-year period of analysis, the net change in water 

storage within the basin is zero (ΔS = 0). This assumption is 

based on visual inspection of four groundwater-level records 

from observation wells within the Pawcatuck-Wood Basin that 

span the period of analysis (wells RI–WEW 522, RI–CHW 

18, RI–SNW 6, and RI–SNW 515). These records indicate no 

long-term upward or downward trends in groundwater levels 

Therefore, equation 3 can be simplified to

SF P = o + ET . (4) A

In this report, the units of measurement are inches per year 

for precipitation and evapotranspiration, cubic feet per second 

for streamflow, and square miles for drainage area. Appropriate conversion factors are used to convert each of these 
Water Budgets for the Example Basin 23

Figure 14. Graph of monthly 

recharge, in inches over the 

drainage basin, calculated 

by the RORA program for the 

Pawcatuck River Basin at 

Wood River Junction, Rhode 

Island, from January 1971 

through December 2000.

units to a single, consistent unit of inches per year for all 

water-budget terms.

Average annual rates of precipitation and streamflow for 

the 30-year period were shown previously to equal 51.8 and 

27.7 in., respectively. Therefore, equation 4 can be used to 

estimate the average annual rate of evapotranspiration from 

the basin (24.1 in/yr), which, because it is the difference 

between observed precipitation and streamflow, is taken to 

be the actual rate of evapotranspiration from the basin. This 

rate differs from a potential rate of evapotranspiration, which 

is the rate that would occur if there was a nonlimiting supply 

of water. Although the GW Toolbox provides computed rates 

of potential evapotranspiration for many weather stations 

(the PEVT dataset described previously), these data are not 

useful for the water-budget analysis. The calculated rate of 

actual evapotranspiration is 46.5 percent of the average annual 

precipitation rate (that is, ET/P), which is very close to the 

value of 44.5 percent calculated by Sanford and Selnick (2012, 

supplemental data table S1) for the entire 295 mi2 Pawcatuck 

River Basin for the same period.

Table 1. Water-budget components estimated for the Pawcatuck River Basin at Wood River Junction, Rhode Island from 1971 

through 2000.

[All water-budget components are given in inches over the 100-square-mile drainage basin. NOAA, National Oceanic and Atmospheric Administration; NCDC, National Climatic Data Center; USGS, U.S. Geological Survey; NWIS, National Water Information System; GW, groundwater; PRCP, 

precipitation dataset]

Water-budget 

component Method of analysis Value (inches)

Precipitation NOAA NCDC precipitation data retrieved with the GW Toolbox (PRCP dataset) 51.8

Streamflow USGS NWIS streamflow data retrieved with the GW Toolbox 27.7

Base flow Average of the six hydrograph-separation methods provided with the GW Toolbox 22.8

Runoff Streamflow minus base flow 4.9

Recharge RORA method provided with the GW Toolbox 26.6

Evapotranspiration, 

total

Method 1: Precipitation minus streamflow 24.1

Method 2: From regression model developed by Sanford and Selnick (2012) and NOAA NCDC 

data retrieved with the GW Toolbox

26.0

Evapotranspiration, 

groundwater

Recharge minus base flow 3.8

Evapotranspiration, 

near surface

Total evapotranspiration (method 1) minus evapotranspiration from the groundwater system 20.3
24 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

In some cases it is not possible to use equation 4 to 

estimate actual ET, and other approaches are needed. Sanford 

and Selnick (2012) developed a regression model to estimate 

actual ET throughout the conterminous United States on the 

basis of climate and land-cover data. Although the estimates 

of ET are improved with the land-cover information, results 

of the Sanford and Selnick (2012) study indicate that climate 

variables—specifically, precipitation and temperature—are the 

most influential in determining ET. Because the precipitation 

and temperature data that are needed for the model are available with the GW Toolbox, use of the model is demonstrated 

here for the Pawcatuck River Basin.

The regression model is expressed as the ratio of ET to P

in the following general form:

ET  τ∆  = Λ , (5)   P τ∆+Π 

The Greek letters reflect different nondimensional parameters: 

Λ for land cover, τ for temperature, Δ for the mean diurnal 

temperature range, and Π for precipitation. Variable Λ is set 

equal to 1 when only climate data are used in the model. The 

three other variables (τ, Δ, and Π) are defined by mathematical equations given in Sanford and Selnick (2012, table 1). 

In practice, the temperature and precipitation variables that 

are needed for the regression model and available with the 

GW Toolbox are: ATEM for mean annual daily temperature 

(variable Tm in Sanford and Selnick, 2012, in degrees Celsius), 

TMAX and TMIN for mean annual maximum and minimum 

daily temperature (variables Tx

 and Tn, in degrees Celsius), 

respectively; and PRCP for mean annual precipitation [variable P, in centimeters, (cm)]. The values of these variables for 

the Kingston weather station for the period from 1971 through 

2000 were shown previously to be: Tm = 49.59 °F (9.77 °C), Tx

= 60.73 ° (15.96 °C), Tn

 = 39.38 °F (4.10 °C), and P = 51.82 

in. (131.62 cm). The ratio ET/P is then found from the equations presented in Sanford and Selnick (2012) to be 0.502; or, 

evapotranspiration was 50.2 percent of precipitation during the 

period from 1971 through 2000. Multiplication of ET/P by the 

estimate of P (51.82 in.) results in an estimated value of ET of 

26.0 in. for the period from 1971 through 2000. For comparison, Sanford and Selnick (2012, supplemental data table S1) 

estimated a value of ET/P of 48.6 percent for the entire Pawcatuck River Basin for the same period.

Three additional water-balance equations can be written 

for streamflow, groundwater recharge, and evapotranspiration 

from the water-budget components estimated previously. The 

first is for streamflow out of the basin:

SFo BF RO , (6) = +

A AA

where

 BF is the average annual rate of base flow 

(groundwater discharge) out of the basin 

(cubic length over time) and

 RO is the average annual rate of surface runoff to 

the stream (cubic length over time).

Using the mean average annual values estimated by the six 

hydrograph-separation methods presented earlier, base flow 

and surface runoff from the drainage area for the period from 

1971 through 2000 are 22.8 and 4.9 in/yr, respectively.

The second equation is for the average annual rate of 

recharge to the water table (R, in/yr). Following the approach 

suggested by Rutledge (1998, p. 39), it is assumed that the 

recharged water either discharges to the stream network as 

base flow or is lost by evapotranspiration from the groundwater system where the water table is close to land surface, such 

as in riparian zones:

BF R = + ET , (7)

A gw

where

 ETgw is the average annual rate of 

evapotranspiration directly from the 

groundwater system, and represents an 

average rate distributed across the entire 

area of the basin (length over time).

Because the rates of recharge (26.6 in/yr) and base flow 

(22.8 in/yr) have been estimated from the hydrograph analyses, ETgw for the 30-year period is calculated from equation 

7 to be 3.8 in/yr. This value is approximately 0.8 to 1.8 in. 

greater than those reported for the Pawcatuck River Basin by 

Barlow (1997) and Barlow and Dickerman (2001), most likely 

because the value reported here is based on six methods used 

to estimate base flow, whereas only the PART method was 

used in the analyses by Barlow (1997) and Barlow and Dickerman (2001). If only the result of the PART method is used 

(24.9 in/yr), a value of ETgw equal to 1.8 in/yr is calculated, 

which is within the range reported by the previous studies.

Finally, the rate of actual evapotranspiration within 

the basin can be broken down into two components, that 

from the groundwater system (ETgw) and the remainder from 

evapotranspiration near land surface and from open bodies of 

water (ETns):

ET , = ET (8) ns + ETgw

where

 ETns is the average annual rate of 

evapotranspiration near land surface, 

including open bodies of water (length 

over time).

Because rates of ET (24.1 in/yr) and ETgw (3.8 in/yr) have 

been estimated previously, ETns is found from equation 8 to be 

20.3 in/yr for the 30-year period.
References Cited 25

Summary

The primary purpose of this report is to provide a set of 

instructions that will allow users of the Groundwater (GW) 

Toolbox to quickly develop skills to use the mapping, dataretrieval, data-inspection, and hydrograph-analysis methods to 

estimate base flow, surface runoff, and groundwater recharge 

for a basin. The report describes six hydrograph-separation 

methods used to determine the groundwater-discharge (baseflow) and surface-runoff components of streamflow—the 

Base-Flow Index (BFI; Standard and Modified), HYSEP 

(Fixed Interval, Sliding Interval, and Local Minimum), and 

PART methods—and the RORA recession-curve displacement method and associated RECESS method to estimate 

groundwater recharge from streamflow data. The GW Toolbox 

provides graphing, mapping, and analysis capabilities using a 

geographic information system driver within a Microsoft Windows environment. The GW Toolbox contains several preprocessed GIS data layers, meteorological data from the National 

Oceanic and Atmospheric Administration National Climatic 

Data Center, and automated tools to retrieve time-series data 

from NWIS.

Each of the hydrograph-analysis methods is based on 

a number of simplifying assumptions that limit their applicability. The methods should be applied to the analysis of 

streamflow hydrographs that reflect contributions from two 

sources: surface runoff in response to a precipitation event and 

groundwater discharge from a single aquifer. The methods are 

intended for application to basins dominated by diffuse areal 

groundwater recharge uniformly distributed over a basin, as 

opposed to focused groundwater recharge such as occurs from 

losing stream reaches. All groundwater recharge within the 

basin discharges to the receiving stream network except that 

amount that is evapotranspired directly from the groundwater 

system (sometimes referred to as riparian evapotranspiration). These hydrograph-separation methods are based on the 

assumption that groundwater discharge to streams is a continuous process, whereas the recession-curve displacement method 

is based on the assumption of episodic recharge in response 

to storms. Figure 1 in the report provides guidance on the 

appropriateness of hydrograph-analysis methods provided 

with the GW Toolbox for application to a particular basin. 

Users should understand the hydrologic conditions in a basin 

to confirm the quantitative results of the hydrograph-analysis 

methods. Execution of multiple hydrograph-analysis methods 

simultaneously helps the user to frame the range of base flow 

and recharge estimates.

The methods provided with this version of the GW 

Toolbox are demonstrated with streamflow data collected at 

the Pawcatuck River at Wood River Junction, Rhode Island, 

streamgage. Using the data-retrieval and data-analysis tools 

within the GW Toolbox, the report provides a tutorial of the 

steps necessary to estimate many of the components of the 

water budget for a hydrologic basin, including precipitation; 

streamflow; base flow; runoff; groundwater recharge; and 

total, groundwater, and near-surface evapotranspiration.

References Cited

Arnold, J.G., and Allen, P.M., 1999, Automated methods for estimating baseflow and ground water recharge 

from streamflow records: Journal of the American Water 

Resources Association, v. 35, no. 2, p. 411–424.

Barlow, P.M., 1997, Dynamic models for conjunctive management of stream-aquifer systems of the glaciated northeast: Storrs, Conn., University of Connecticut, Ph.D. 

dissertation, 256 p.

Barlow, P.M., and Dickerman, D.C., 2001, Numerical-simulation and conjunctive-management models of the HuntAnnaquatucket-Pettaquamscutt stream-aquifer system, 

Rhode Island: U.S. Geological Survey Professional Paper 

1636, 88 p., http://pubs.usgs.gov/pp/pp1636/.

Bent, G.C., Zarriello, P.J., Granato, G.E., Masterson, J.P., 

Walter, D.A., Waite, A.M., and Church, P.E., 2011, Simulated effects of water withdrawals and land-use changes 

on streamflows and groundwater levels in the Pawcatuck 

River Basin, southwestern Rhode Island and southeastern 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2009–5127, 254 p., http: //pubs.usgs.gov/

sir/2009/5127/.

Cervione, M.A., Jr., Richardson, A.R., and Weiss, L.A., 1993, 

Low-flow characteristics of selected streams in Rhode 

Island: U.S. Geological Survey Water-Resources Investigations Report 93–4046, 16 p., http://pubs.er.usgs.gov/publication/wri934046.

Delin, G.N., Healy, R.W., Lorenz, D.L., and Nimmo, J.R., 

2007, Comparison of local- to regional-scale estimates 

of ground-water recharge in Minnesota, USA: Journal of 

Hydrology, v. 334, nos. 1–2, p. 231–249.

Eckhardt, K., 2005, How to construct recursive digital filters 

for baseflow separation: Hydrological Processes, v. 19, 

p. 507–515.

Eddy-Miller, C.A., Constantz, Jim, Wheeler, J.D., Caldwell, 

R.R., and Barlow, J.R.B., 2012, Demonstrating usefulness 

of real-time monitoring at streambank wells coupled with 

active streamgages—Pilot studies in Wyoming, Montana, and Mississippi: U.S. Geological Survey Fact Sheet 

2012–3054, 6 p., http://pubs.usgs.gov/fs/2012/3054/.

Halford, K.J., 2008, Discussion on “Update on the use of the 

RORA program for recharge estimation,” by Al Rutledge: 

Ground Water, v. 46, no. 1, p. 10–11.

Halford, K.J., and Mayer, G.C., 2000, Problems associated 

with estimating ground water discharge and recharge from 

stream-discharge records: Ground Water, v. 38, no. 3, 

p. 331–342.
26 U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data

Hamon, W.R., 1961, Estimating potential evapotranspiration: Journal of the Hydraulics Division, Proceedings of 

the American Society of Civil Engineers, v. 87, no. HY3, 

p. 107–120.

Healy, R.W., 2010, Estimating groundwater recharge: Cambridge, United Kingdom, Cambridge University Press, 

245 p.

Healy, R.W., Winter, T.C., LaBaugh, J.W., and Franke, O.L., 

2007, Water budgets—Foundations for effective waterresources and environmental management: U.S. Geological Survey Circular 1308, 90 p., http://pubs.usgs.gov/

circ/2007/1308/.

Institute of Hydrology, 1980a, Research report, v. 1 of Low 

flow studies: Wallingford, United Kingdom, Institute of 

Hydrology, 42 p.

Institute of Hydrology, 1980b, Catchment characteristic 

estimation manual, v. 3 of Low flow studies: Wallingford, 

United Kingdom, Institute of Hydrology, 27 p.

Lim, K.J., Engel, B.A., Zhenxu, Tang, Choi, Joongdae, Kim, 

K.S., Muthukrishnan, Suresh, and Tripathy, Dibyajyoti, 

2005, Automated WEB GIS based hydrograph analysis tool, 

WHAT: Journal of the American Water Resources Association, v. 41, no. 6, p. 1407–1416.

MapWindow, 2013, MapWindow GIS Open Source Project: 

MapWindow Web site, accessed December 13, 2013, at 

http://www.mapwindow.org/.

Nathan, R.J., and McMahon, 1990, Evaluation of automated 

techniques for base flow and recession analyses: Water 

Resources Research, v. 26, no. 7, p. 1465–1473.

Pettyjohn, W.A., and Henning, Roger, 1979, Preliminary estimate of ground-water recharge rates, related streamflow and 

water quality in Ohio: Columbus, Ohio State University, 

Water Resources Center Project Completion Report 552, 

323 p.

Piggott, A.R., Moin, Syed, and Southam, Chuck, 2005, A 

revised approach to the UKIH method for the calculation of 

baseflow: Hydrological Sciences, v. 50, no. 5, p. 911–920.

Risser, D.W., Gburek, W.J., and Folmar, G.J., 2005, Comparison of methods for estimating ground-water recharge and 

base flow at a small watershed underlain by fractured bedrock in the eastern United States: U.S. Geological Survey 

Scientific Investigations Report 2005–5038, 31 p.

Rorabaugh, M.I., 1964, Estimating changes in bank storage 

and ground-water contribution to streamflow: International Association of Scientific Hydrology Publication 63, 

p. 432–441.

Rosenberry, D.O., and LaBaugh, J.W., 2008, Field techniques 

for estimating water fluxes between surface water and 

ground water: U.S. Geological Survey Techniques and 

Methods, book 4, chap. D2, 128 p., http://pubs.usgs.gov/

tm/04d02/.

Rutledge, A.T., 2007, Update on the use of the RORA program for recharge estimation: Ground Water, v. 45, no. 3, 

p. 374–382.

Rutledge, A.T., 1998, Computer programs for describing the 

recession of ground-water discharge and for estimating 

mean ground-water recharge and discharge from streamflow 

records—Update: U.S. Geological Survey Water-Resources 

Investigations Report 98–4148, 43 p., http://pubs.usgs.gov/

wri/wri984148/.

Rutledge, A.T., 2000, Considerations for use of the RORA 

program to estimate ground-water recharge from streamflow 

records: U.S. Geological Survey Open-File Report 2000–

156, 44 p., http://pubs.usgs.gov/of/2000/ofr00-156/.

Rutledge, A.T., and Daniel, C.C., III, 1994, Testing an automated method to estimate ground-water recharge from 

streamflow records: Ground Water, v. 32, no. 2, p. 180–189.

Rutledge, A.T., and Mesko, T.O., 1996, Estimated hydrologic 

characteristics of shallow aquifer systems in the Valley and 

Ridge, the Blue Ridge, and the Piedmont Physiographic 

Provinces based on analysis of streamflow recession and 

base flow: U.S. Geological Survey Professional Paper 

1422–B, 58 p.

Sanford, W.E., Nelms, D.L., Pope, J.P., and Selnick, D.L., 

2012, Quantifying components of the hydrologic cycle in 

Virginia using chemical hydrograph separation and multiple regression analysis: U.S. Geological Survey Scientific 

Investigations Report 2011–5198, 152 p., http://pubs.usgs.

gov/sir/2011/5198/.

Sanford, W.E., and Selnick, D.L., 2012, Estimation of evapotranspiration across the conterminous United States using a 

regression with climate and land-cover data: Journal of the 

American Water Resources Association, v. 49, issue 1, p. 

217–230, http://dx.doi.org/10.1111/jawr.12010.

Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic unit maps: U.S. Geological Survey Water-Supply 

Paper 2294, 63 p., http://pubs.usgs.gov/wsp/wsp2294/.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP—A computer 

program for streamflow hydrograph separation and analysis: 

U.S. Geological Survey Water-Resources Investigations 

Report 96–4040, 46 p., http://pubs.er.usgs.gov/publication/

wri964040.
References Cited 27

Stonestrom, D.A., and Constantz, Jim, eds., 2003, Heat as 

a tool for studying the movement of ground water near 

streams: U.S. Geological Survey Circular 1260, 96 p., 

http://pubs.usgs.gov/circ/2003/circ1260/.

U.S. Environmental Protection Agency, 2013, BASINS (better 

assessment science integrating point and non-point sources): 

U.S. Environmental Protection Agency Web site, accessed 

December 13, 2013, at http://water.epa.gov/scitech/datait/

models/basins/index.cfm.

U.S. Geological Survey, 2013, Web interface: U.S. Geological Survey National Water Information System Web site, 

accessed December 13, 2013, at http://waterdata.usgs.gov/

nwis/.

Wahl, K.L., and Wahl, T.L., 1995, Determining the flow of 

Comal Springs at New Braunfels, Texas, in Proceedings of 

Texas Water 95, August 16–17, 1995, San Antonio, Tex.: 

American Society of Civil Engineers, p. 77–86.

Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Chief

Office of Groundwater 

U.S. Geological Survey

411 Federal Center 

Reston, VA 20192 

http://water.usgs.gov/ogw
Barlow and others—U.S. Geological Survey Groundwater Toolbox, A Graphical and Mapping Interface for Analysis of Hydrologic Data User Guide—TM3–B10

ISSN 2328-7055 (online)

http://dx.doi.org/10.3133/tm3B10